Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.
Tunable laser sources have a number of important applications including spectroscopy, interferometry, astronomy and optical communications. Previous methods for providing wavelength tuneability in lasers include adjusting the laser cavity length and introducing selective feedback into the cavity. However, these designs suffer from unstable output signals due to effects such as spatial hole burning, mode competition and polarization drift. The output stability of a laser source is often the limiting factor in its use for real world applications.
US Patent Application Publication number US 2003/0210727 entitled “Narrowband Filter Method and Apparatus” is a publication of an earlier application by the inventors of the present application. US 2003/0210727 discloses a narrowband filter device which utilizes a relative resonant wavelength shift in the phase response of a Gires-Tournois resonator. Orthogonal signal components are incident onto a Gires-Tournois resonator at slightly different angles to introduce a relative phase difference between the orthogonal components at a particular wavelength. This phase difference is utilized to manipulate and filter a certain signal component at the particular wavelength. This filtered wavelength is returned at the same polarization state.
While US 2003/0210727 suggests use of such a filter in a tunable laser, it does not significantly overcome the shortfalls mentioned above in relation to output stability. Tunable laser sources providing more stable outputs are desired.